Dilute hydrochloric acid , Lanthanum solution, Standard preparations, and Blank solution—Prepare as directed in the test for Limit of calcium under Magnesium Carbonate.

Test preparation— Transfer 250 mg of Magnesium Hydroxide, previously dried, to a beaker, add 30 mL of Dilute hydrochloric acid, and stir until dissolved, heating if necessary. Transfer the solution so obtained to a 200-mL volumetric flask containing 4 mL of Lanthanum solution, dilute with water to volume, and mix.

Procedure— Proceed as directed in the test for Limit of calcium under Magnesium Carbonate: the limit is 1.5%.

Blank solution— Transfer 3.0 mL of nitric acid to a 50-mL volumetric flask, and dilute with water to volume.

Thallium internal standard 20 ppb— [note—Use this solution only if an ICP–MS instrument is used. This internal standard is added in-line via a mixing block between the sample probe and the spray chamber.] Dilute 20.0 mL of a commercially prepared thallium ICP standard solution (1000 ppb) with water to 1 L.

Dilute nitric acid— Dilute 2.0 mL of nitric acid with water to 100 mL.

Standard stock solution 100 ppb— Prepare this solution fresh every two months. Quantitatively dilute an accurately measured volume of a commercially prepared lead ICP standard (1000 ppm) with Dilute nitric acid to obtain a solution containing 10 ppm of lead. Further dilute this solution with Dilute nitric acid to obtain a solution containing 1000 ppb of lead. Transfer 10.0 mL of this solution to a separate 100-mL volumetric flask, add 2.0 mL of nitric acid, and dilute with water to volume.

Standard solutions— Prepare these solutions fresh weekly. [note—The concentrations specified below are recommended if an ICP–MS instrument is used. If an ICP–AES instrument is used, the concentrations of the Standard solutions may be modified to adapt to the working range of the instrument.] Transfer 5.0 mL of the Standard stock solution 100 ppb to a 50-mL volumetric flask, add 3.0 mL of nitric acid, and dilute with water to volume (Standard lead solution 10 ppb). Transfer 5.0 mL of Standard lead solution 10 ppb to a 50-mL volumetric flask, add 3.0 mL of nitric acid, and dilute with water to volume (Standard lead solution 1 ppb).

Test solution— [note—The concentration specified below is recommended if an ICP–MS instrument is used. If an ICP–AES instrument is used, the concentration of the Test solution may be modified to adapt to the working range of the instrument.] Accurately weigh about 0.25 g of Magnesium Hydroxide. Cautiously add 3.0 mL of nitric acid, and mix until the sample is dissolved. Accurately transfer this solution to a 50-mL volumetric flask, and dilute with water to volume.

Procedure (see Plasma Spectrochemistry 730)— The inductively coupled plasma–mass spectrometer (ICP–MS) is equipped with a quadrupole mass spectrometer and an ion detector maintained under vacuum. The instrument should read all isotopes for lead (206, 207, and 208 amu) and the thallium internal standard (205 amu), and should report the total lead content using the most naturally abundant isotope at 208 amu. Alternatively, lead could be determined using an inductively coupled plasma–atomic emission spectrometer (ICP–AES) by measuring the emission at 220.353 nm, with the settings optimized as directed by the manufacturer. [note—To minimize matrix interference when using an ICP–AES instrument, it is recommended that the method of standard additions be used.]